The present invention relates to a process for producing a thermal barrier coating, in which organometal complexes of zirconium and at least one stabilizing element selected from the group of the alkaline earth metals or rare earths are provided as starting substances, the starting substances are evaporated by heating and the coating gases which are generated are transported to a component to be coated, which is heated at a deposition temperature, where they are broken down so that a layer is deposited.
Electron beam physical vapor deposition (EB-PVD) processes, in which the substances that are to be deposited on the metallic component, such as, for example, zirconium oxide, are vaporized in a high-vacuum environment using an electron beam, are conventionally used for the production of thermal barrier coatings. On account of the considerable introduction of energy, a thin, molten zone is formed, from which the substances are vaporized and, in a condensation reaction, are deposited on the surface of the component. The layers produced in this manner have a columnar structure which tolerates expansion, is better able to withstand alternating temperature stresses and results in a prolonged service life.
Drawbacks of these processes are the extremely high installation costs for the electron beam gun, for the generation of the high vacuum, for the vacuum chamber and for the partial pressure control. Furthermore, those surfaces of the component that are not directly visible cannot be coated or can only be insufficiently coated during the coating cycle.
European Published Patent Application No. 0 055 459 describes a process for producing oxide layers by chemical vapor deposition (CVD), in which complexes derived from diketones, such as, for example, acetylacetonate complexes, are mixed with steam in order to oxidize the metals contained in the complexes and are deposited on a substrate. In the process, the substrate is heated in various applications to temperatures of between 350xc2x0 C. and 800xc2x0 C. The thicknesses of the deposited layers are in the range between 3.6 and 34 xcexcm. The use of steam as a carrier gas has proven imperative, since oxygen does not enable either reproducibility or deposition to be achieved.
International Published Patent Application No. WO 94/21841 describes a flame CVD process for applying inorganic layers to substrates, in which mixed oxides, such as yttrium-stabilized zirconia, are deposited at flame temperatures of between 300xc2x0 C. and 2800xc2x0 C. and pressures that are well above ambient pressure. The starting substances for the coating gases are passed into the flame and, in a flame CVD process of this type, cannot be heated with a defined temperature cycle and transported to the substrate.
In conventional processes for producing thermal barrier coatings by means of chemical vapor deposition (CVD), it has only been possible to produce very thin layers with a low deposition rate and without a columnar structure, which layers also present poor adhesion and, moreover, contain relatively large quantities of undesirable carbon impurities. Relative to industrial use, the selection of the starting substances is of particular importance, since they must not be too expensive and they must be available in sufficient quantities.
It is an object of the present invention to provide a process for producing a thermal barrier coating in which a thermal barrier coating with sufficient layer properties and a columnar structure is produced as inexpensively as possible.
According to one example embodiment of the present invention, the starting substances are heated, at a process pressure of 0.5 to 50 mbar, to at most 250xc2x0 C. so that the coating gases are formed, and the coating gases are transported to the component to be coated, the surface of which is heated at a deposition temperature of between 300xc2x0 C. and 1100xc2x0 C.
Thermal barrier coatings that contain zirconium oxide and, for example, yttrium oxide, may be produced with a sufficiently large layer thickness of approximately 25 to 1000 xcexcm using the process that is based on the chemical vapor deposition (CVD) principle. Moreover, the thermal barrier coatings produced in this manner have a suitable crystal structure and morphology and required layer properties. In terms of their ability to withstand alternating temperature stresses, the layers are comparable to those produced using the EB-PVD process. A further advantage is that, unlike in the electron beam physical vapor deposition (EB-PVD) process, the scattering force of the process means that even those surfaces of the component to be coated that are not directly visible may be coated.
Organometal complexes, which are derived from diketones, of zirconium and at least one stabilizing element selected from the group consisting of the alkaline earth metals or rare earths are provided as starting substances, since with these components the coating gases are completely broken down or burnt when they come into contact with that surface of the component that has been heated to deposition temperature. Moreover, they have the advantage over alkoxides that they are not sensitive to hydrolysis and are therefore easier to handle.
Furthermore, the coating gases may be mixed with a carrier gas, such as, for example, oxygen or a mixture of oxygen and argon.
In a further example embodiment of the process according to the present invention, the coating gases or the coating gases and the carrier gas may be transported to the component to be coated, which is arranged in a receptacle, in an admission system that has been heated to at most 250xc2x0 C.
The process may be performed at a low process pressure of 0.5 to 50 mbar, in order that the coating gases or the coating gases and the carrier gas are transported as quickly as possible, so that their residence time in the hot zone produced by the thermal radiation of the component or substrate that has been heated to the deposition temperature is as short as possible and to minimize vapor phase reactions.
Yttrium, lanthanum, calcium, magnesium or cerium may be provided as the stabilizing element from the group consisting of the alkaline earth metals or rare earths, since they are not excessively expensive with regard to process costs and, furthermore, are available in sufficient quantities for industrial use.